Precise regulation of pyrocarbon coating

Abstract

Deposition of a pyrocarbon coating of precise thickness onto one or more substrates being levitated along with a bed of particles in a fluidized bed coating enclosure is accomplished by varying the amount of hydrocarbon supplied to the bed as a part of an upward levitating flow to compensate for changes which are detected in the size of the fluidized bed. Increases and decreases in the size of the bed are detected by monitoring either the differential pressure above and below the bed or the weight of the bed, and compensating changes are made to return the bed to its desired size by changing the flow rate of hydrocarbon. For example, when a growth in bed size is detected that is not attributable to an aberration in either particle supply or withdrawal, the amount of hydrocarbon being supplied to the bed is decreased so as to cause the size of the bed to gradually return to its desired value.

What is claimed is:  
     
       1. A method of controlling the rate of deposition of pyrocarbon onto a substrate while it is being levitated in an enclosure along with a fluidized bed of particles in order to produce a precise thickness of deposit of pyrocarbon of uniform crystalline properties, which method comprises 
       establishing a bed of particles in fluidized condition in a coating zone within an enclosure along with at least one substrate to be coated, all of which are levitated by supplying an upward flow of a gaseous atmosphere which comprises an inert gas,  
       heating said bed of particles and said substrate to a pyrolysis temperature in said coating zone and supplying said levitating gaseous atmosphere in a form that includes a mixture of a hydrocarbon component and an inert gas component, each of which components is supplied to said coating zone at a certain initial flow rate, such that pyrolysis of said hydrocarbon occurs causing pyrocarbon to be deposited upon surfaces of said substrate and upon said particles in said coating zone,  
       monitoring either (a) the weight of said fluidized bed, or (b) the differential pressure between a location within or below said bed and a location above said bed, to determine changes that occur either in said weight or in said differential pressure, and  
       adjusting the amount of said hydrocarbon component being supplied as a part of said upward coating flow based upon changes determined to have occurred either in said weight or in said differential pressure so as to compensate for such changes and thereby precisely regulate the rate of deposition of pyrocarbon over time and thereby produce a precise thickness of pyrocarbon coating of uniform crystalline properties upon said surfaces of said substrate during a coating run lasting for a definite period of time.  
     
     
       2. A method according to claim  1  wherein said hydrocarbon is propane. 
     
     
       3. A method according to claim  2  wherein said inert gas constitutes at least about 50 volume percent of said gaseous atmosphere which also includes an organic silicon compound, which compound decomposes at said pyrolysis temperature to deposit silicon carbide alloyed with said pyrocarbon. 
     
     
       4. A method according to claim  2  wherein said temperature is maintained between about 1250° C. and about 1400° C. 
     
     
       5. A method according to claim  4  wherein said propane is maintained in said levitating gaseous atmosphere between about 20% and about 60% by volume. 
     
     
       6. A method according to claim  4  wherein the flow rate of said propane being supplied to said enclosure to create said upward flow is increased or decreased within a range of not more than about 10% from said initial propane flow rate. 
     
     
       7. A method according to claim  6  wherein said inert gas is supplied at a flow rate which is increased or decreased to inversely correspond to each change in flow rate of propane. 
     
     
       8. A method according to claim  1  wherein said differential pressure is monitored and wherein said pressure is measured at a location within said fluidized bed which is within the lower one-half of said bed, based upon total volume of said bed. 
     
     
       9. A method according to claim  8  wherein said pressure within said bed is measured via a port through which inert gas is constantly flowed and wherein said pressure is monitored above said bed at a location at the end of a passageway extending into said enclosure. 
     
     
       10. A method according to claim  1  wherein particles are continuously added to said bed at a rate of a certain number of grams per minute and particles are withdrawn from said bed at a constant rate over time throughout said coating run of said substrate. 
     
     
       11. A method according to claim  10  wherein said particles being added are zirconium dioxide having a density greater than the density of said pyrocarbon being deposited. 
     
     
       12. A method according to claim  1  wherein the weight of said fluidized bed is monitored by: 
       initially weighing at least a portion of said enclosure wherein said bed is established such that said bed is weighed along with said at least a portion of said enclosure;  
       periodically reweighing said portion of said enclosure at desired intervals so as to determine periodic changes in the overall weight thereof over said intervals; and  
       determining changes in weight of said bed over said intervals based on said periodic changes in said overall weight.  
     
     
       13. A method according to claim  12  wherein said determining step includes the step of subtracting a deposit factor from said overall weight, said deposit factor being an approximation of the weight of pyrolytic carbonaceous material deposited onto said portion of said enclosure following said initial weighing so that compensation is made therefor in determining any said change in said weight of said bed. 
     
     
       14. A method according to claim  13  wherein said determining step includes the step of measuring the weight of the particles withdrawn from said bed which are included in said overall weight. 
     
     
       15. A method according to claim  14  wherein said overall weight is measured using at least one load cell. 
     
     
       16. A method of controlling the rate of deposition of pyrocarbon onto a plurality of heart valve component substrates while being levitated in an enclosure along with a fluidized bed of particles in order to produce a precise thickness of deposit of pyrocarbon of uniform crystalline properties, which method comprises 
       establishing a bed of particles in fluidized condition in a coating zone within an enclosure along with the substrates to be coated, all of which are levitated by supplying an upward flow of a gaseous atmosphere which comprises an inert gas,  
       heating said bed of particles and said substrates to a pyrolysis temperature in said coating zone and supplying said levitating gaseous atmosphere in a form that includes a mixture of a hydrocarbon component and an inert gas component, each of which components is supplied to said coating zone at a certain initial flow rate, such that pyrolysis of said hydrocarbon occurs causing pyrocarbon to be deposited upon the surfaces of said substrates and upon said particles in said coating zone,  
       monitoring said fluidized bed within said coating zone to determine changes that occur which are indicative of a change in the size of said fluidized bed, and  
       periodically changing the flow rate of said hydrocarbon component from said initial flow rate to periodically adjust the amount of said hydrocarbon component being supplied as a part of said upward coating flow, based upon each said change so determined to have occurred, as to compensate for such changes and thereby precisely regulate the rate of deposition of pyrocarbon over time and thereby produce a precise thickness of pyrocarbon coating of uniform crystalline properties upon the surfaces of said substrates during a coating run lasting for a definite period of time.  
     
     
       17. A method of controlling the rate of deposition of pyrocarbon onto a plurality of heart valve component substrates while being levitated in an enclosure along with a fluidized bed of particles in order to produce a precise thickness of deposit of pyrocarbon of uniform crystalline properties, which method comprises 
       establishing a bed of particles about 1000 microns or less in size in fluidized condition in a coating zone within an enclosure along with the substrates to be coated, all of which are levitated by supplying an upward flow of a gaseous atmosphere which comprises an inert gas,  
       heating said bed of particles and said substrates to a pyrolysis temperature in said coating zone and supplying said levitating gaseous atmosphere in a form that includes a mixture of a hydrocarbon component in the form of methane, ethane, propane, butane acetylene, propylene or a mixture thereof and an inert gas component, each of which components is supplied to said coating zone at a certain initial flow rate, such that pyrolysis of said hydrocarbon component occurs causing pyrocarbon to be deposited upon the surfaces of said substrates and upon said particles in said coating zone,  
       monitoring said fluidized bed within said coating zone to determine changes that occur which are indicative of a change in the size of said fluidized bed, and  
       changing the flow rate of said hydrocarbon component by an amount not greater than about 10% of said initial flow rate to periodically adjust the amount of said hydrocarbon component being supplied to said coating zone as a part of said upward coating flow, based upon each said determined change, to compensate for such changes and precisely regulate the rate of deposition of pyrocarbon over time and thereby produce a precise thickness of pyrocarbon coating of uniform crystalline properties upon the surfaces of said substrates during a coating run lasting for a definite period of at least about 2 hours.